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tinted Mar. 18, 1941 ERROR CHECKING TELEGRAPH SYSTEM Arjen Bakker, The Hague, and Hendrik Cornelis Antonie van Duuren, Wassenaar, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application October 1, 1937, Serial No. 166,804

A In Great Britain May 9, 1933 14 Claims.

This invention relates to error checking telegraph systems. It has particularly to do with apparatus for detecting errors in telegraph signals and for checking the operation of a printer so that printing of a character in response to a mutilated signal will be prevented. Another feature of the invention is the provision of means for transmitting a control signal from the receiving station to the transmitting station whereby and transmitter is caused to repeat a signal which had been mutilated.

The instant application is a continuation in part of our application, Serial No. 724,804, which was filed May 9, 1934, and which matured into Patent 2,119,196, dated May 31, 1938.

In operating a type-printing telegraph system on a signaling channel in which interference may occur, for example, a radio channel, a dilculty is encountered which is not present in systems wherein the actual signal is recorded. The difficulty is that a wrong signal cannot be recognized as such. In systems in which the actual signal is recorded it is occasionally possible for a skilled telegrapher to recognize Wrong signals.

The object of the invention is to provide a type printing telegraph system in which the unnoticed reproduction of signals or signal elements Wrongly received may be avoided.

This object is attained by transmitting the signals by means of double current. The working elements of the signals are transmitted by means of working or marking current and the resting elements by means of resting or spacing current. In the receiving circuit an error-detecting device is provided whereby the presence of disturbing impulses or the dropping out of signal elements is made recognizable.

The construction of the printing telegraph system so as to achieve the object of the invention results in a marked increase in eiciency.

According to the invention, the desired effect may be attained by the provision of a device having means controlled by the received working current and means controlled by the received resting current, which means so co-operate that extraneous and mutilated signal elements are made recognizable in the receiving apparatus either by preventing said signal elements from becoming eiective in the receiving apparatus or by setting in operation an interference circuit.

The invention may be employed in all known type printing telegraph systems. It .has particular advantages in two-way communication between two stations, and in systems in which, for the purpose of eliminating interference, each (Cl. 17E-69) signal element is sent out several times by the transmitting device and is received as many times by the receiving device. These advantages will be explained more fully hereinafter.

'I'he invention will be explained with reference 5 to the accompanying drawings showing several constructional examples. In these drawings:

Figure 1 shows an error detecting device constructed according to the invention.

Figures 2 and 3 relate to modifications of the l0 error detecting device.

Figure 4 shows a printing telegraph system as employed for two-way communication between two stations, and adapted for carrying out th invention.

Figures 5-9 relate to three constructional examples for systems in which the signals or signal elements are transmitted and received a plurality of times. Y

In Figure v1, the error detecting device cornprises two relays, one of which is acted upon by the marking signal and the other by the spacing signal from the transmitter. The tongues of these relays co-operate in such a manner that when the reception is not disturbed so that Vone tongue rests in the Working position and the other in the resting position. the received signal elements are passed on to the printer portion of the receiving apparatus.

If however, the reception is disturbed then consequently the tongues both lie in the working position, or both in the resting position. Although the mutilated signals are passed on, they are, nevertheless, designated as erroneous by means of an additional circuit which is rendered operative in response to the occurrence of the interference. It is also possible to prevent the actuation of the printing apparatus in response to multilated signals.

When a carrier wave is used for the signaling channel, then it is preferable that this carrier be differentially modulated. One modulating frequency designates a marking element and another ,modulating frequency designates a spacing element. The two sources of modulating frequencies are transmitted alternately, one to produce the marking signal and the other to produce the-spacing signal. They are shown at I and 2, and 3 is the tongue of the transmitting relay. However, this circuit arrangement of the transmitting station is merely illustrative and is not'essential for the invention.

At the receiving end, means are provided for detecting the successively transmitted marking and spacing frequency currents. These signal currents are thereupon directed into separate circuits by known means such as filters l, 5. The marking current and the spacing current are now fed to rectiilers 8 and 1 respectively and these rectiilers act upon relays 8 and 9 respectively. The tongue of relay 8 rests upon the contact r when the rectifier B does not receive any signal voltage, and is moved to the contact w when a current of marking frequency is rectified by the rectiiler 6. The tongue of relay 9 correspondingly rests upon its contact r when the rectifier 1 does not receive any spacing signal voltage or any interfering voltage, and is moved to the contact w upon receipt of either of these voltages. The filter 5, however, is designed to reject the marking frequency and consequently the armature of relay 9 would remain on its idle contact r during reception of a marking signal which is rectified by the rectifier 6.

The normal reception of unmultilated signals is such, therefore, that the tongue of relay 8 moves to its contact w, while the tongue of relay 9 is held on its contact r during reception of a marking impulse. Furthermore, the tongue of relay 8 is held on its contact r, while the tongue of relay 9 is moved to its contact w upon reception of a spacing impulse.

If, however, the received signal element is disturbed, either on account of the occurrence of an additionalv current impulse or due to the absence of a transmitted current impulse, both relay tongues are shifted to corresponding positions, that is both working or both resting. The cause may be a fading condition or any atmospheric disturbance.

The tongues of the relays 8 and 9 are electrically connected together and are also connected by a lead I to the printer apparatus. The resting contact of the relay 8 and the working contact of the relay 9 are connected together and to an interference relay II, while the working contact of the relay 8 and the resting contact of the relay 9 are both connected to a source of current I2. One side of the relay II and one side of the.

source of current I2 are grounded.

With this connection, the interference relay II receives current when the tongues of bothrelays 8 and 9 occupy the resting position r (interference through fading) or when they both occupy the working position w, (interference through an additional current impulse). In the event of undisturbed reception, on the other hand, the relay II does not receive any current.

The printing apparatus connected to the lead III receives marking current when the relay 8 is in the working position w and the relay 9 is in the resting position r. It does not receive any current when the relay 8 is in the resting position and the relay 9 is in the working position, and it receives marking current when both relays occupy the same position (that is to say in the case of interference). In the last case, however, the received signal may be indicated as an error by means of the interference relay I I, orin any other manner. This relay, therefore, indicates erroneous signals with absolute certainty.

It is obvious that the interference relay I I may be arranged to act upon the printer in many different ways. Thus, for example, a blank or another interference signal, such as a special printed sign, may be given. Furthermore, the starting of the printer may be prevented in systems in which the code signal is followed by a locally initiated printer actuating impulse. The practical execution depends upon the particular telegraph apparatus employed, and is not an essential feature of the invention.

According to a modification of the invention the occurrence of a wrong signal element may be rendered recognizable, by the employment of means responsive to such an erroneous signal for intercepting the working or resting current as supplied to the printer. The printer will then function as when "spacing instead of printing erroneous characters. 'This feature is of considerable importance when using uniform length code signals in printing telegraph systems, as will be shown more clearly hereinafter.

Figure 2 illustrates how the relays 8 and 9 may be so employed that the disturbed signal element is kept away from the printer. As will be seen from this figure, the printer doesnot receive any current when the tongues of the relays 8 and 9 are both in the unoperated position r or both in the operated position w, because the current sources I2 and I3 are of such potentials and the resistance R is so related to that of the printer circuit that the conductor I8 would then be substantially at ground potential. If, on the other hand, the received signal is undisturbed, the telegraph printer receives either positive marking current or negative spacing current from the current sources I2 and I3 respectively.

Figure 3 shows another constructional form. In the event of interference-free reception, the anode current of one of the rectiflers 8 or 1 flows through one or the two windings of the relay 98, so that the tongue of the relay rests against one of the two contacts. In the event of interference, current either flows through the two coils in opposite directions or else both coils remain unenergized. In both cases the tongue of the relay which always returns to the middle position, remains in this middle position, so that the printer does not receive any signal.

The invention as used in two-way communication renders it possible to repeat the'transmission of a certain signal by one of two stations, if this signal is not correctly received at the other station. If, therefore, a signal is mutilated during transmission, it is repeated and'if it is not mutilated it will not be repeated.

This method is preferably carried out in such a manner that the reception of the mutilated signal by station A results automatically in the transmission by station A of a monitoring signal back to station B whereby the signal is caused to be repeated by the latter station. Furthermore the reception of an interference-free signal results automatically in a monitoring signal being sent back, whereby the succeeding signals of a message are caused to be transmitted in regular course.

Figure 4 will serve for explanation of the foregoing. In this figure, a construetonal form of the system is shown dagrammatically and as adapted to the use of the well known 5-unit code. The two-way or cross trailic takes place between two stations A and B, which are substantially constructed in the same way. For the sake of simplicity, the operation of the system will be described as when a communication originates at station B and is received at station A. -So, although the transmitting and receiving apparatus has been shown as if located at only one station, the transmitting elements may be considered generally applicable to station B, and the receiving apparatus will be referred to as appropriate to station A. Where the description relates to so-called answer-back signaling, however, the reference numerals for parts at different stations have been given prefix letters A and B in the speciiication to indicate at which of the two stations the part in question is caused to function in the case of transmission of a message from station B to station A, the answer-back signal being transmitted in the reverse direction.

The receiving distributor Al, A2 of station A runs synchronously with the transmitting distributor B3 of station B. and the receiving distributor of station B runs synchronously with the transmitting distributor of station A. 'The two distributors in each station may be coupled together mechanically. It is immaterial whether synchronism is obtained by means of distributors moving continuously or discontinuously. In the present instance an installation with continuously rotating distributors has been assumed.

The correction current impulses for maintaining synchronism oi' the distributors in the trans- .mitting station and the receiving station are sent out by the transmitting station B through distributor segments 63 and 64, and are received by the receiving station A on segment 65. I'he correction magnet CM of station A is connected to a protective device 66. The latter operates in the manner already described in the foregoing and may be so constructed, for example, that if an interference occurs, while the receiving distributor brush is traversing segment |02, the correction magnet of station A will not operate. Premature correction is, therefore, avoided.

The five elements of each signal are transrnitted by station B, the procedure being to set the contact tongues |09 either up to obtain plus potential from the source 24, or down to obtain minus potential from the negative source which is normally available when relay 2| is unoperated and its armature 25 rests on the back contact 23. Segments 26 to 30 inclusive are traversed by brush 403 for the purpose of timing the code elements of each ve unit signal. The signals are received at station A on segments 46 to 50 whence they are transferred to the proteotive device 88 and signal storing device 89. The protective device serves to ascertain whether one or more of the ve signal elements received has been disturbed. If none of the ve elements is disturbed, the interference relay l remains unenergized, and the actuating magnet M of the telegraph recording apparatus |03 receives a 'starting current impulse by way of the back contact I4 of relay |5 (to which negative battery potential is applied) and segment |0| of the distributor AI, which segment is grounded when traversed by brush 40|. The telegraph printer |03 then becomes operative, receiving the signal elements stored up in storage relays 89 and causing suitable characters to be printed or recorded in the usual manner.

The transmitting distributor B3 comprises two rings of contact segments so arranged that each segment in one ring may be connected by thev bridging brush 403 to a corresponding segment in the other ring. 'Ihe contact brushes 40|, 402, 403 are preferably adapted to be rotated in synchronism by a common driving motor.

The receiving relays 8 and 9 are the same as those shown in Figs. 1 and 2. They cooperate with relays 04 and |05 which have tongues springbiased toward their negative contacts.

The relay group 88 comprises ve polar relays,

one ior each oi the five signal units constituting one complete signal. Five other relays are included in the group 89, each being paired with one of the relays of group 88.

An undisturbed marking signal causes relay 8 to move its armature against its left hand contact which supplies potential to relay |04, assuming that relay 9 does not operate. Similarly an undisturbed spacing signal causes relay 8 to move its armature against its left hand contact which supplies potential to relay |05, assuming that relay 8 does not operate. An interfering impulse is responded to by both relays 8 and 9, so that both relays |04 and |05 remain unenergized and their armatures both receive negative potential. A fading condition renders both relays 8 and 9 unresponsive, and the armatures of relays |04 and |05 areboth held by their springs against their negative contacts. The negative potential actuates different relays of groups 88 and 89 in accordance with the operation of the grounded brushes 40| and 402, moving the, armatures of relays to the right. Considering each separate pair of relays of the groups 88 and 89, their armatures are interconnected, and connections therefrom are carried to the code-element responsive devices in the receiving printer |03.

In connection with Fig. 4 it has been stated that the correction current impulses necessary for maintaining synchronism between the distributors of the transmitting and the receiving station are sent out by the transmitting station B on segments 63 and G4 and are received by the receiving station A on segment |02. According to well-known practice, synchronism is maintained by slightly shifting the distributor phase of the receiving station with respect to its own driving phase by means of a correction magnet which is energized as soon as a certain phase difference between the distributor of the transmitting station and that of the receiving station occurs.

From Fig. 4 it will be seen that the correction current impulses transmitted by the contact segments 63 and B4 comprise a positive (spacing) Vand a negative (marking) current impulse in will receive rst a positive `current impulse and then a negative current impulse. Under normal conditions this positive current impulse is not long enough to produce operation of the relay |08. If, however, the distributor phase of the receiving station is slightly in advance of the distributor phase of the transmitting station the positive current impulse will last long enough to cause the relay |08 to be energized, so that the correction magnet CM is actuated and operates to restore the desired synchronism in the manner outlined above. In Fig. 4 the correction magnet is shown in this operative position.

If the correction current impulses are received disturbed, an actuation of the correction magnet CM does not occur, as stated in connection with Fig. 4. This is for the purpose of preventing premature correction. The result of such a disturbed correct-ion current impulse received by the contact segment |02 is that the tongue of the relay |05 is brought to its right-hand contact. In this position the relay |08 receives a negative current impulse so that the circuit of the correction magnet CM is interrupted.

Details o! the answer-back system In order that the transmitting station B may be controlled either to repeat a signal or to "go ahead with the transmission of a succeeding signal, the receiving station A is provided with means for reporting either the mutilation of received signals or the reception of unmutilated signals. In the first place, as has been shown, the relays of group in combination with the relays of group 80 with which they are individually paired provide for energizing relay I whenever a signal of marking frequency is accompanied by a disturbance, also whenever there is a fading condition. The negative potential from conductor ||0 under these conditions is fed through one of the armatures of a relay in the group 09 to a corresponding armature of its twin relay in group 08 and thence to the relay I5, one terminal of which is grounded. On energization of relay I5 the actuating magnet M in the receiving printer |03 is disabled at the moment when brush 40| traverses segment |0I.

Relay |5 also closes its contacts a and b against front stops one of which supplies negative potential to the left hand contact of relay |01, thus preparing this relay when it is closed to feed a negative pulse through relay I1. Such a pulse takes place when brush 40| traverses segment |06, causing the armature of relay I1 to be moved to its right hand contact. This contact supplies negative potential to the transmitting distributor segment I8 in station A. When the transmitting distributor brush 403 bridges the two segments I8 a repeat signal is initiated by relay 61 and 3, this repeat signal having a marking frequency characteristic. Station B responds to such a signal when its receiving distributor brushes 40| and 402 traverse segments I9.

If the transmitting station B receives this repeat signal correctly, then relay 404 moves its armature to the left, while relay 405 moves its armature to the right. The armature of relay 405, being supplied with negative potential, feeds the same to the armature of relay 404. Relay 404, however, throws the armature to the left so that this negative potential does not reach relay 2|. The armature 25 of relay 2|, therefore, remains on its back contact which is supplied with negative potential, suitable for initiating marking impulses of a code signal. Such a signal may be either the repetition of the previous signal or the transmission of a subsequent signal, as the case may be.

Since the answer-back signal decribed in the foregoing lparagraph is of a marking frequency characteristic (calling for repetition of the signal) the transmitter starting magnet SM at station B is open-circuited by the placement of the armature of relay 405 to the right. Under this condition no new signal can be transmitted but the previous signal will be repeated, since marking signal potentials may be derived from the negative back stop against which the armature 25 of relay 2| rests.

When complete fading of the signal occurs relay 35 and relay Il will both be operated. Repetition of the signal by station B is then called for, the same as in the case of reception of a mutilated Signal.

When a code signal has been correctly received at station A some one or more of the relay pairs in groups 80 and 09 will be so actuated as to feed negative potential from the conductor IIO to relay 35, thus closing its contacts 34 and feeding positive potential to the left hand contact of relay |01.

Relay |01 in the device 33 remains normally closed until signal interference occurs. Therefore, relay I1 is actuated by this positive potential and throws its amature I3 against the left hand or positive contact. Station A then transmits a go ahea signal over its transmitting segment I0, this signal being characterized as a spacing frequency. Station B receives this signal by actuating relay 3 to feed potential to relay |05 which is responded to by the device in such manner that the armature of relay 405 is moved to the left, while the armature of relay 404 is moved to the right. Under these conditions relay 2| still remains unactuated but the starting magnet SM in the transmitter 22 at station B causes the subsequent signal to be sent out.

By the starting of the transmitting apparatus, the transmitting tongues |09 are connected to plus and minus voltages which correspond to the next signal, so that this signal is now transmitted in accordance with the passage of brush 403 successively over the segments 26 and 30 inclusive. In station A this signal is received during the passage of brushes 40| and 402 over segments 46 to 50 inclusive. The process described in the foregoing is thus repeated for sending successive signals so long as no interference occurs.

Relays 2| and 35 on being energized remain in the energized position for one complete revolution of the distributor. At the end of this revolution they are either de-energized or re-energized, according to the current impulses delivered thereto.

The purpose of relay 2| is to cause repetition of the monitoring signal in case that signal as transmitted from station Ato station B becomes mutilated. In such a case let it be assumed that the device 2'0 at station B operates so that the armatures of both relays 404 and 405 are thrown to the right. Negative potential will then be supplied to relay 2| and cause it to close its contacts 25 for supplying positive potential to segment 3| lon the transmitting distributor of station B. Station B will then transmit a special signal of spacing frequency characteristic back to station A which is received on segment 32 when the brush 402 traverses the same. This special signal then actuates relay |01 to open the circuit for relay I1. Relay I1, however, will remain locked in the position to which it was last energized and the answer-back signal will be repeated. Also five spacing current impulses are transmitted from station B. The receiving apparatus of station A does not respond to these ilve spacing impulses.

Normally ,the answer-back acknowledgment signal to which device 33 responds consists of an impulse of marking frequency, which frequency maintains the armature of relay |01 against its left hand contact. Like other signals, however, the answer-back acknowledgement signal may itself become mutilated, in which case relay |01 moves its armature to the right so that relay I1 remains in position tov repeat the answer-back signal.

It is obvious that a certain time is necessary in order to transmit a monitoring or answer-back signal by means of the transmitting apparatus of the receiving station. This signal can only be transmitted when the ilve elements of the transmitted signal have been received andexamined. Some time elapses, therefore, before the transmitting apparatus of the transmitting station can start again. The time of waiting between the successive transmissions offthe same or diiierent signals may. ii necessary, be iilled in with the signals of another telegraph system which employs the same transmission channel. This may be effected by means of contact members, which bring about in known manner a suitable time distribution, so that the individual telegraph systems can employ the transmission channel successively.

It'is furthermore obvious that the connections as shown in FlgA may also be constructed otherwise within the scope of the invention. Thus, for example, the devices i8, 38. 89, 20 and 33 are not coniined to a deiinite form of construction. This already follows from the discussion oi' the protective devices shown in Figs. 1, 2 and 3. It is also not necessary to employ special interference relays I5, 2l, 35. The circuits which are controlled by these relays may also be opened or closed by theprotective devices themselves.

A further possible means of combating interference in a type printing telegraph system, according to the invention, resides in the application of the known principle, namely, that interference may be eliminated up to a certain extent by repeated transmission and reception of all signal elements and signals.

The repeating systems known for this purpose l are generally only capable of combating interference of a certain character, that is `to say, either interference due to additional current impulses, or interference due to rent impulses.

For telegraph systems in which both types of interference may occur, means are usually provided such that the repeatedreception of the marking current element or spacing current element results in the formation `of a marking current signal or a spacing current signal, as the case may be. l Y

This property of the known systems obviously shows their great disadvantage, namely, that the repeated reception of marking or spacing current elements, which are not caused by a repeatedly transmitted signal but by interference, can lead to the formation of an erroneous, but nevertheless unnoticeable signal.

If, on the other hand, the principle of repetition is employed in combination with the present invention the undisturbed reception f a single transmission is suflicient in order to render possible the reproduction of the correct signal in the actual receiving apparatus, because each interference is indicated with absolute certainty. If none of the transmissions is received undisturbed this is also rendered recognizable with certainty in the receiving apparatus. 'I'he unnoticed reproduction of a wrong signal is, therefore, impossible.v Several constructional forms .will be described hereinafter, by way of example, of a testing device according to the invention, which is adapted to be provided'in the receiving installation between :the protective device and the actual receiving apparatus, and which renders it possible to ascertain Whether, in the repeated transmission, at least one of the received signals is correct. If this is so, this signal is passed on further to the telegraph apparatus so that' the latter is set in operation. If, however, none of the signals passed on by the protective device is undisturbed, a monitoring signal is supplied in any manner to the telegraph apparatus, or the operation of the said apparatus is prevented.

The testing device must satisfy the condition of being able to ascertain and store the difference between three different units which can be the absence of curgiven by the protective device, that is to say, between a marking signal, a spacing signal, and an interference signal (or zero signal). Various means may be employed for this purpose, and the following are worthy of note:

l. Condensers which are adapted to receive and re-transmit subsequently the difference between a positive current impulse. a negative current impulse and no current impulse.

2. Relay circuits which are adapted to receive and re-transmit the difference between a positive current impulse, a negative current impulse and zero current impulse, or between current impulses of different intensities, or between direct current and alternating current.

3. Relays in combination with mechanical devices for receiving, storing and retransmitting the three different units.

In Fig. 5 is shown a testing device employing a distributor or any other equivalent device which is driven synchronously. in rhythm with the repeated transmission, with the repeating device of the transmitter, which supplies successively to a condenser the quantities of electricity, given by the protective device during the repeated transmission of each signal and which subsequently employs the charge of this said condenser for actuating the receiving apparatus.

It has been assumed in the figure that each signal element is received three times and hence can be tested three times.

The current impulses which are given by a rent in order to give a resting signal, a working signal, or an interference signal, are supplied to the brush 25. Contacts 2|, 22 and 23 of a distributor (contact disc, contact drum, or the like) are closed just at the moments at which the repeated signal elements are received in the brush 25. It depends upon the construction of the distributor whether the contacts 2|, 22 and 23 are separated from one another or are combined to form a single contact. 'I'he construc- Ition of the distributor is, of course. optional, the only condition being that it shall operate synchronously with the repeating system of the transmitter.

When any one of the contacts 2| is closed, the first current impulse given by the protective device has the opportunity of charging or not charging an associated condenser 26. The contact 22 does the same for a second current impulse and the contact 23 for a third current impulse. Since the three current impulses belong to the same signal element, the condenser 26 may be charged only in a certain direction either positively or negatively, or may remain uncharged. Current impulses of opposite directions are not possible in the same signal element, since the protective device either passes on the received signal correctly or not at all, and a wrong transmission is impossible.

After the lapse of the third repetition, the condenser is hence charged certainly in the correct direction, if any undisturbed signal element has been received at least once by the protective device. If this is not the case, the condenser is not charged.

After the condenser has functioned to receive three times a correct current impulse the distributor closes the contact 24, whereby the condenser can discharge through a polarized relay `2" and a non-polarized relay 28. The tongue of the polarized relay 21 is applied to the resting contact or the working contact. a resting or working current impulse being passed on to the telegraph apparatus by the energizing ofy the relay 28. Ii' the condenser 26 has no charge, the non-polarized relay 28 is'not energized. An interference signal can be then transmitted to the telegraph apparatus by the resting contact of this relay.

The non-polarized relay hence serves to distinguish current and zero current from one another, and the object ot the polarized relay is to transfer the current direction (working or resting current) to the receiving apparatus. In other respects. it is clear that the circuit in which the condenser 26 is discharged may be constructed in diierent ways. 'This construction depends, for example. upon the nature of the telegraph apparatus connected thereto.

When, after vthe repeated transmission of a vdenite signal element, the condenser 26 has performed its action, it is in a position to receive the current impulses of a following signal element. The explanation given above has been limited to one signal element merely for the sake of simplicity, for example, to the irst signal element of a signal comprising live elements.

The manner in which the condenser, after exerting its eiect can, if necessary, be completely discharged for the sake of security, and themanner.- when 'the nature .of thel connected telegraph apparatus demands, in which the'relay 28 can be maintained energized for some time, for

example, by means of a holding winding, and a special contact on theV distributor, after the condenser has already been discharged, will n ow be explained.

Fig. 5l comprisesthree parts. A is a protective device which receives all incoming signal impulses, B is a repeating device including a testing device, and C is the receiving apparatus.

The protective vdevice A is of the kind shown in Fig. 3. No current will be delivered by the tongue of the relay 88 on reception of a disturbed current impulse, as said tongue will then remain in its neutral or middle position. I

The device B comprises a distributor having two contact rings I' and I each consisting of a number of contact-segments. These segments will vbe connected to a 'slip ring ,(not shown)y one l after' another when the contact brush shown at the top is moved in the direction ot thearrow. The receiving distributor is running synchronously with the transmitting distributor of It has not been shown in v the distant station. Fig. how this synchronism is maintained since this has already been explained in connection with Fig. 4.

The device B further comprises a testing device having tive polarized relays 21, five nonpolarized relays 28 and five ycontacts 24, one for each of the five units constituting a complete signal. The circuit arrangement of the relays 21 and 28 and the contacts 24 will be explained later.

The receiving apparatus C is shown as a'startstop teleprinter, the construction of which is known.

It will be seen that the left-hand distributor section is provided with nfteen contact segments. This means that each five-unit signal is transmitted and received three times in succession. The contact segments taking up the first transmission are denoted 2|, those taking up the second transmission are denoted 22, and

those taking up the third transmission are denoted 23.

The contact segments 2| are each connected to a condenser 26, and circuits are extended therefrom to responding contact segments of the groups 22 and 23. Thus, it will -be clear that during the repeated reception of a certain fiveunit signal each of the condensers 26 will have the chance of getting changed three times.

The right-hand distributor section I" is provided with ve contact segments 24 and a contact segment |23. Each of the contact segm-ents 24 is connected through a polarized relay 21 and a non-polarized relay 28 to one of the condensers' 26. When the repeated reception of the live signal units has been finished, so that the condensers 24 are or are not charged, `the contact brush shown at the top of I" is in the direction of the arrow and the five contact segments 24 are connected to the grounded brush 25 one after another, thereby enabling the ve condensers 26 to be discharged over the corresponding relays 28 and 21. Those condensers having a certain charge when their contact segments 24 are passed by the contact brush will cause the corresponding relays `28 and 21 to be energized and those condensers not Abeing charged will leave their relays 28 and 21 unaiected. The energization of those relays 26 through which current flows will be maintained by the closure of a holding contact I2 I which is connected in series with an auxiliary winding |30. The right-hand terminal of this auxiliary winding |30 is connected to the tongueof the polarized relay 21, so that the current flowing through the auxiliary winding is always of the same direction as that flowing through the main winding.

From the above it follows that if all the condensers 26 have been charged during the repeated reception oi.' the live-unit signal all relays 28 and 21 will be energized, and if only part of the condensers have been charged only part of the relays 28 and 21 will be energized. As explained in connection with Fig. 5, a charged condenser 26 means that the signal unit in question has been correctly received at least once during been received correctly if all or only part of the relays 28 and 21 will be energized when the contact segments 24 are passed by the contact brush.

Now, each of the relays 28 is provided with an `auxiliary contact |24 and all these contacts |24 are connected in series between a relay |22 and the contact segment |23 of the distributor. The contact |23 is closed when all contact segments 24 have been passed by the contact brush, and when this happens the relay |22 will or will not be energized, according to whether all the auxiliary contacts |24 or only part of them have been closed by the relays |28. In the former case the circuit of the starting magnet M of the receiving apparatus C is closed by the tongue of the relay |22, and in the latter case it is not. If the starting magnet M is energized, the voltages supplied to the tongues of the five polarized relays 21 are taken oi by the teleprinter C one after another and they are supplied to the printing magnet shown which controls the printing mechanism. If, on the other hand, not all the relays 28 have been energized, the relay |22 will not be operated and starting of the teleprinter C will be prevented.

time but to print an additional disturbance mark when the reception of the signal has been mutilated.

To the contact segment |28 there is further connected a slowly operating relay |25 which is energized somewhat later than the relay |22 and which serves for the purpose of interrupting the holding circuits of the ilve relays 2l (by opening the contacts |26 and |21). so that these relays are de-energized and restored to the normal condition. The system is then ready to take up a fresh five-unit signal. l e

Fig. 6 shows diagrammatically the construction of another testing device in which a number of relay chains is employed. It has been assumed that each signal element is transmitted twice. The relay chains are closed one after the other by a distributor or an equivalent device, driven synchronously with the repeating device of the transmitting end, at moments in which the individual current impulses or zero current impulses, given by the protective device during the repeated transmission of -the same signal element, arrive at the testing device. The relay chains are constructed in such manner that testingof the received current impulses is carried out and the receiving apparatus is brought into action when at least one of the repeated transmissions is received undisturbed.

The relay chains of the testing device each comprise a non-polarized relay 4| with a holding i contact 36, and in series therewith a polarized relay 42. The contact 3|. of the distributor is closed at the moment at which the protective device operates onv the iirst reception of thetwice transmitted signal element. If the reception is undisturbed the protective device gives rise to a working current or a resting current and the nonpolarized relay is energized and is kept energized by means of its holding contact 36. The polarized relay 42 applies its tongue in accordance with the direction of the received current impulse, either to the resting contact or to the working contact, so'that "this current impulse is passed on by Way of the contact 3l of the nonrpolarizecl relay di to the second relay chain, whereby this second relay chain which was originally connected to the leal 35 by the contact 39 is disconnected from the protective device.

lit the moment of the reception ci the secondA transmission the contact 32 of the distributor is closed, the non-polarized relay 4t is then energized and is maintained energized by Way oi the contact 3d, while by means of the polarized relay tt a current impulse is supplied to the telegraph apparatus. The contacts tt and tt or the distributor have the object of short-circuiting the holding Winding of the relays 4l andtt and or returning the circuit into the original state, after the respective relay chains have completed their action.

if an interference occurs in the first transmission of the signal element, the protective device does not transmit any current impulse. In this case the non-polarized relay 4i is not energized, the result of which'isfthat the second relay chain remains connected to the lead 35. On the second reception Yof the repeated signal element, the second relay chain then comes. into action. It this reception is undisturbed the relays 43 and 44 are energized, and the telegraph apparatus receives a corresponding current impulse. If, however, the second transmission also is disturbed, the non-polarized relay 43 is not energized and an interference signal is supplied to the telegraph apparatus by means of the contact 4l and a contact, not shown, on the distributor. If the contacts 3|, 32, 33, 34 of the distributor are closed in known manner in the correct sequence, the first relay chain will be ready to receive a new current impulse, while the telegraph apparatus is still being fed by the second relay circuit.

' A more detailed description of Fig. 6 follows: .Five pairs of relays are shown, each pair being appropriate to one of the live elements of a riveunit code signal.

The current impulses are received from a protection device and they are transferred to the vtesting device shown, by way of the conductor 35.

The distributor includes contact segments 3| and 32. By means of the contact brush 2i shown at the top of this distributor', the segments 3| and 33 are connected yto ground one after another. The distributor is running synchronously with the vtransmitting distributor of the distant station. Just how this synchronism may be maintained has already been explained in connection with Fig. 4.

The five second relay pairs of Fig. 6 are shown below the five rst relay pairs. 'Each of these second relay pairs comprises a non-polarized relay 43, a polarized relay 44 and a contact 32. The conductors leaving the tongues of the relays 44 of the second relay pairs are each connected to one of the contacts of a. teleprinteriC which is of the kind already described in connection with Fig. 5.

The collaboration between the relays 43 of the five second relay pairs and the teleprinter C of Fig. 6 is the same as that between the relays 2B and the teleprinter of Fig. 5. The same reference numbers have been used in this connection, so that it may be easily checked how in Fig. 6 the teleprinter C is operated and how the relays H3 are restored to their normal condition when the repeated transmission of the five signal units has been finished.

It is to be noted that in each of the five rst relay pairs the contact 33 is arranged in iront of the contact tl in the distributor i. possible because the only requirement to be met by the contact tt is that it shall have been closed, in order to restore the corresponding re,

lay ti to its normal condition at the time that the contact ti is operated.

Fig. '7 shows a third construction of the testing device. This operates substantially in a rnechanical manner.

At the transmitting end of the telegraph system, a perforated strip ti'i, on which the signals to be transmitted are tired, is passed through three automatic transmitters ti, tit, til in succession. These transmitters are so connected to a distributor tt that they are connected in succession to the transmission channel.

The perforated strip runs through the three transmitters and the elements of each signal are transmitted automatically three times. The time intervals between the successive transmissions are quite optional.

At the receiving end are three so-called reperforators til, 55, 58. These are located behind a distributor t I, which follows the protective device 60 and is driven synchronously with the dis- This is tributor 58 oi the transmitting station. The receiving distributor connects the three reperforators to the transmission channel at the moments at which the current impulses originating from the transmitting station set the protective device 5@ into operation.

A strip 5S, the speed oi which is equal to that of the perforated strip 5l of the transmitting station, is passed through the three reperforators.

A given signal is transmitted on the rst reception to the repertorator 56 and this reperforator produces a perforation in the strip for each signal element or, if one or more of the signal elements are received disturbed, does not produce perforations for these elements.

In order that the reperf-orators can show the difference between the three diierent units (positive current impulse, negative current impulse and no current impulses), the perforations caused, for example, by a positive or resting current impulse are cifset in the direction of the strip relativelygto the perforations caused by a negative or working current impulse. The relative position is shown in Fig. 8. This iigure relates to the case in which each signal comprises ve units. In the transverse direction of the strip there is room for ve periorations. The perfo'- rations for the resting units are not situated, however, on the same transverse line as those on the working units. It has, furthermore, been assumed that the signal which should be received by the reperiorator 54 consists of three resting units and two working units.

At the moment at which the repeatedly transmitted signal is received for the second time, the distributor switches in the second reperforator 55. At this moment the strip 58 has been advanced until the perforations caused by 54 on the rst reception are situated exactly opposite the perforating members of the reperforator 55. If now on the rst reception no perforation was produced at certain points of the strip 58, due to interference of one or more of the ve signal elements, the reperforator can correct this fault, unless an inter-ference again occurs just at this moment. 'I'he same applies for the third reperforator 5B which comes into operation on the third reception of the transmitted signal.

This testing device also operates in such a manner that the receiving apparatus is set into operation when the protective device passes on at least once a signal (or signal element) which has not been mutilated by interference.

When, due to the occurrence of interference, none of the three reperforators is successful in producing a correct perforation, there is in the strip a non-perforated place which is shown in Fig. 9 and which renders recognizable the wrong reception with absolute certainty.

After the strip has left the-last reperforator it is passed, for example, to an apparatus which converts the perforations to type printing. This type printing apparatus may be so constructed that in the absence of a perforation it produces an interference signal.

In addition to the provisions on the lines set forth in the foregoing of a protective device and a testing device, the invention may also be employed in telegraph systems with repeatel transmission of all signals by combining a protective device with two known repeating systems, one for the working current and the other for the resting current.

We claim:

1. In a radio telegraph system having both a transmitter and a receiver at each of two stations. means for applying frequency-discriminating modulations to the marking and spacing signals respectively, means at the receiver of one station, including a monitoring relay, responsiverto the reception of disturbed signals for initiating an answer-back signal for transmission to the second station where said marking and spacing signals originated, and means at said second station responsive to the reception of said answerback signal for causing the transmitter to repeat the signals that were disturbed in transit.

2. In a radio telegraph system a transmitter for sending marking and spacing impulses having frequency-distinguished modulations, a receiver having a plurality of lters for selectively accepting said impulses, relay means responsive in one way to the reception of an impulse having a marking-frequency characteristic, and responsive in a diderent way to the reception of an impulse having a spacing frequency characteristic, a device operable by said relay means for detecting the presence of mutilated signals, a repeater associated with said transmitter for causing each signal to be transmitted a plurality of times, means responsive to the operation of said mutilated-signal-detecting device for so controlling the operation of said receiver that it is set in operation to record only the un-mutilater signals, and further means responsive to the operation of said detecting device when a given signal and each of its repetitions are all mutilated for sending a repeat-signal back to said transmitter.

3. In a two-way radio telegraph system, transmitting and receiving devices at each of two stations, a code signal transmitting distributor and a code signal receiving distributor at each station, a monitoring device associated with each receiving distributor, means responsive to the reception of a mutilated code signal for enabling said monitoring device at one station to transmit a repeat-signal to the other station, and means at the other station operative in response to said repeat signal for causing the transmitting distributor thereof to repeat the code signal that was previously mutilated.

4. In a two-way radio telegraph system, transmitting and receiving devices at each of two stations, a code signal transmitting distributor and a code signal receiving distributor at each station, a monitoring device associated with each receiving distributor, means responsive to the reception of a correct code signal for enabling said monitoring device at one station to transmit a go-aheadsignal to the other station, and means at the other station for causing the transmitting distributor thereof to continue the transmission of code signals of a message without repetition in response to the reception of successive go-aheadsignals.

5. In a radio telegraph system, a transmitting station and a receiving station, a code signal keyer at the transmitting station, a signal storage device associated with signal responsive means at the receiver, means at the transmitter for normally so actuating said keyer as to cause each code signal combination to be repeated at least once, means including a printing telepraph instrument at the receiver for recording the reception of a character Whenever one at least of the repeated codesignals is stored Without mutilation, means operablev for transmitting an answer-back signal from the receiving station to the transmitting 'station only when all of the repeated code signals as stored are mutilated, and means at the transmitting station responsive to said answer-back signal for initiating a keying operation thereby to again repeat the code signal that was mutilated.

6. In a radio telegraph system, a transmitting station and a receiving station, a code signal keyer at the transmitting station, a signal storage device associated with signal responsive means at the receiver, means at the transmitter for normally so actuating said keyer as to cause each code signal combination to be repeated at least once, means including a printing telegraph instrument at the receiver for recording the reception of a character whenever one at least of the repeated code signals is stored without mutilation, and means operable atthe receiving station whenever all of the repeated code signals as stored are mutilated for sending an error designating signal back to saidtransmitting station.

7. In a two-way telegraph system, a plurality of stations, each station having a code impulse transmitter and a code impulse receiver, an error detecting device operable by each receiver upon reception oi' a mutilated signal, means operable under control of saidV error detecting device for causing the transmitter at the station where said mutilated signal is received to send an answerback signal tothe station of origin of signals including said mutilated signal, and means associated with the receiver and transmitter at said station of origin and operable in response to the reception ofsaid answer-back signal for causing a repetition of the signal that was mutilated.

8. In a two-way telegraph system a plurality of stations including station A and station B, each station having a transmitter and a receiver, means for causing the transmitter at station A to normally send .traffic-signals consisting of code impulses, means to arrest the normal operation of said -transmitter and to cause a repetition of a given traiilc signal, an error-detecting device operable under control of the receiver at station B upon reception of a mutilated trailic signal, means for causing the transmitter at station B to send one of two answer-back signals selectively. the first of said answer-back signals being in response to the operation of said error-detecting device when a mutilated signal is received, and the second of said answer-back signals being in response to the reception of each un-mutilated trame signal, and means operable by the receiver at station A in response to answer-back signals from station B for selectively controlling the said means at station A whereby the transmitter thereat is caused to continue normal sending o! .traffic .signals or to repeat the same.

' 9. In a two-way telegraph system of the character described having two stations, each station having apparatus for transmitting and receiving current impulses, a protective device associated with the receiving apparatus at one station operable upon the reception of a disturbed signal for sending an answer-back signal to the transmitting station, means at the transmitting station operable upon reception of the answer-back signal for repeating the disturbed signal, said protective device also being operable upon the recepton oi.' an undisturbed signal for sending another answer-back signal to the transmitting station, means at the transmitting station oper- 'able upon receiving the last-mentioned signal for effecting the transmission of the next signal, and means at the transmitting station operable upon the reception of an answer-back signal of the ilrst mentioned type for stopping the trans. mission of the normal signals and for transmitting a special signal. i

10. In a telegraph system, a transmitter having means for sending code signals over a channel of communication, each of said code signals being composed of marking and spacing elements of distinguishable electrical characteristics, a receiver responsive to said code signals and also responsive to static and fading conditions by which communication over said channel is iniluenced, means included in said receiver for discriminating between the reception of code signals which are mutilated by said static and fading conditions, and those which are not, a printer operable only under control oi the received unmutilated code signals, a monitoring transmitter connected to said receiver and operable to send back a special signal in response to the reception oi' a mutilated code signal, a monitoring receiver connected to the ilrst said transmitter, and means operable by .said monitoring receiver in response to the reception of said special signal for causing the iirst said transmitter to repeat the code signal that was mutilated.

1l. A telegraph system in accordance with claim 10 and including means in said monitoring receiver for discriminating between the reception of special signals which are mutilated by said static and fading conditions and those which are not, and means operative when one of said special signals becomes mutilated for causing said monitoring transmitter to repeat the same.

l2. In a telegraph system, a transmitter having means for sending code signals over a channel of communication, each of said code signals being composed of marking and spacing elements of distinguishable electrical characteristics, a receiver responsive to said code signals and also responsive to static and fading conditions by which communication over said channel is influenced, means included in said receiver for discriminating between the reception of code signals which are mutilated by said static and fading conditions, and those which are not, a printer operable only under control of the received unmutilated code signals, a monitoring transmitter connected to said receiver and operable .to send back either of two special signals, one of which has the signiilcance of requesting repetition and the other of which implies that the first said transmitter is to go ahead, a monitoring receiver connected rto the first said transmitter, and means operable by said monitoring receiver in f response -to said special signal for so controlling the first said transmitter that code signals initially mutilated will be repeated and the transmission of unmutilated code signals will proceed' uninterruptedly.

13. In a two-way telegraph system, two terminal stations A and B, each of said stations comprising a transmitter and a receiver, a uniform length code signal sender and a code-responsive printer at each station, means operable by each said receiver upon reception oi.' unmutilated code signals for actuating its printer, said means being also operable upon reception of a mutilated code signal for initiating an answerback signal, a distributor at each station, one of said distributors being maintained in synchronism with the other, transmitting segments on the distributor of station A corresponding to receiving segments on the distributor of station B, and vice versa, certain of said segments being appropriate to the elements of the uniform length code signals, at least one segment oi each distributor being appropriate to the sending function o! the answer-back signal and at least one segment o! each distributor being appropriate to the receiving function ot the answer-back signal. a relay operable by the nrst said means at station B for impressing an answer-back signal on the sending segment appropriate thereto, means connected to the answer-back receiving segment at station A for producing a response capable of causing station A to repeat its previously transmitted code signal. a corresponding relay at station A and corresponding repeat-control means at station B.

14. A two-way telegraph system in accordance with claim 13 and having means for sending and receiving two diiIerently characterized answerbaci: signals, one of said answer-back signals being sent and received after reception ot each.

unmutilated code signal and being elective to permit continued transmission ot code signals by the station at which said answer-back signal is received, and the other ot said answer-hack signals'being elective to set back the code signal sender, thereby in cause repetition of a signal that was mutilated when initially sent.

ARJEN BAKKER. HENDRIK CORNELIS ANTONIE VAN DUUREN. 

